Implantation of the blastocyst is a critical event in embryogenesis but little is known about the molecular events regulating the process. This grant investigates the function of a maternally active gene, Hoxa-10, that plays a role in implantation, and attempts to place Hoxa-10 in the molecular pathway regulating implantation. Mice deficient in this gene have an implantation defect characterized by a reduced vascular permeability response, an impaired uterine decidual reaction and a high rate of embryo resorption. This phenotype is similar to that observed in prostaglandin synthase-2 knockout mice. Hoxa-10 deficient mice exhibit altered expression of prostaglandin E2 receptors EP3 and EP4 suggesting that their phenotype may be due to interruption of the prostaglandin pathway and that Hoxa-10 may directly regulate EP3 and EP4. Preliminary data show that Hoxa-10 expression in the uterine stroma is strongly regulated by estrogen and progesterone which together reproduce the endogenous pattern of antimesometrial Hoxa-10 stromal expression. This grant will test the model, suggested by these data, that estrogen and progesterone regulate the expression of Hoxa-10, which in turn is required for proper prostaglandin E2 receptor expression. The Hoxa-10 implantation defect will be further characterized using markers for attachment, implantation and decidualization. Steroidal regulation of Hoxa-10 during implantation will be addressed by experiments in ovariectomized mice and in tissue culture, and by the use of biochemical and transfection strategies to search for and identify estrogen and progesterone response elements in the Hoxa-10 gene and the Hoxa cluster. Whether EP3 and EP4 are downstream targets for Hoxa-10 regulation and whether regulation is direct will be tested. Finally, an adenoviral vector or transgenic mice will be used to evaluate the effects of Hoxa-10 gene transfer into the uterus of deficient mice to confirm that these molecules are targets for Hoxa-10 regulation in vivo and to examine the consequences of Hoxa-10 misexpression on implantation.